Many commercial post-combustion carbon dioxide (CO2) capture plants use chemical absorption processes with amine-based (e.g., mono-ethanolamine (MEA)) solvents. Others, such as the Benfield process, use carbonates (hot potassium carbonate solution) as a CO2 scrubber solvent. In a typical process, flue gas contacts the scrubber solution in an absorber. The solution selectively absorbs the CO2. CO2-laden scrubber solution is then transferred to a stripper. In the stripper, the CO2-rich solution is heated to release almost pure CO2. The CO2-lean solution is then recycled to the absorber. Concerns about degradation and corrosion have kept the solvent strength relatively low (typically 20-30% by weight) in practical systems, resulting in relatively large equipment sizes and energy requirements and high solvent regeneration costs.
Adding a CO2 capture and compression system to a power plant reduces its overall thermal efficiency significantly, by one estimate up to 24%.
The efficiency reduction is due to the additional parasitic energy load from the CO2 capture system. The parasitic load can be broken down into three components:                (i) energy (steam) to break the chemical bonds between the CO2 and the amine and to raise the temperature of the amine solution to the operating temperature of the stripper (˜60% of parasitic load),        (ii) CO2 compression (˜33%) for pumping; and        (iii) energy (electricity) to push the flue gas through the absorber (˜5%).        
Existing amine-based liquid chemical absorption systems have a number of disadvantages including high parasitic steam loss due to solvent regeneration, sensitivity to sulfur oxides and oxygen, solvent loss due to vaporization, and high capital and operating costs.
An alternate process for CO2 capture is the use of solid sorbents. Solid sorbents may have advantages because of potentially less energy requirements compared to solvent capture, as the heat capacity of the solid carrier is several times lower than the water in the MEA-based solvent. Solid sorbents for low temperature carbon capture and storage include those that are carbon-based, zeolites, supported amines and carbonate-based. Each of these classes of sorbents has advantages and disadvantages. Carbon-based sorbents, which would fall in the physical adsorbent category, have a low energy for regeneration (<10 kJ/mol CO2). However, the CO2 capacity is also low, and more sorbent must be heated during regeneration. Zeolites rely on their structure to act as a molecular sieve for gases. While zeolites exhibit superior capability to remove CO2 from dry simulated flue gas, in the presence of moisture, their adsorption capacity is significantly diminished because of preferred adsorption of water vapor.
Other solid sorbents that have been investigated are supported amines. Most of these materials contain approximately 40-50 weight percent amines; polyethyleneamine is a specific example. The substrates include silica, clay and other high surface area supports. Heat of regeneration for the best amine-based sorbents are between 2000 and 3500 kJ/kg CO2 (˜90-150 kJ/mol CO2) and working capacity is between 3-9 weight percent. All the materials exhibit a slow loss in capacity, in part, to reaction with SO2 to form heat-stable salts, but also due to physical loss of the active component.
The reaction inherent to this process is as follows:Na2CO3(s)+CO2(g)+H2O(g)→2NaHCO3(s)ΔHr=−135 kJ/mole of CO2[1]
Sodium carbonate captures CO2 in the presence of water vapor to form sodium bicarbonate at temperatures around 60° C. By performing a moderate temperature swing to 120 to 140° C., the bicarbonate decomposes and releases a CO2/steam mixture that can be converted into a pure CO2 stream by condensation of steam. There are however, significant challenges with processes based on dry solid carbonate sorbents:    1. The CO2 sorption process above is strongly exothermic and requires significant cooling. Reaction equilibrium is negatively impacted if adsorption reactor temperatures are not controlled.    2. The regeneration (reverse) reaction is strongly endothermic and has a large energy penalty (130-140 kJ/mole of CO2).    3. The released water vapor from the regeneration step has to be condensed to separate it from the CO2. This energy of condensation needs to be recovered and re-used to reduce the overall energy penalty.    4. The CO2 loading capacity of the solid sorbent must be increased to lower the large solids handling and circulation requirements.